Add instructions for regenerating `websites-with-shared-credential-backends.json`

[Sameesunkaria]

quirks/websites-with-shared-credential-backends.json contains a lower fidelity version of the data in quirks/shared-credentials.json and quirks/shared-credentials-historical.json. It must be regenerated using tools/convert-shared-credential-to-legacy-format.rb whenever those files are changed.

This information was not captuted in any of the current documentation, therefore CONTRIBUTING.md has been updated to include it. The workflows we run on each PR have been updated to verify that quirks/websites-with-shared-credential-backends.json is up-to-date.

Overall Checklist

• [x] I agree to the project's Developer Certificate of Origin
• [x] The top-level JSON objects are sorted alphabetically
• [x] There are no open pull requests for the same update

[Sameesunkaria]

Test commit with a failing workflow: https://github.com/apple/password-manager-resources/commit/8d8bb864295516c13ea28d3262b9f66674675a8c Job: https://github.com/apple/password-manager-resources/actions/runs/4777862787/jobs/8493882020